When 1 Bitcoin is worth somewhere around $100 (in 2010 dollars), I think it'll be time for the client to switch to millicoins (e.g. .001 BTC), so if I'm buying a paperback book it costs 50 milliBTC instead of .05 BTC.
We arrived.
And now...?
The client is capable of displaying the balance in mills, but has to be set to do so, as that is not the default.